feat: implement professional HTML ↔ Markdown conversion for group editing

- Add marked and turndown libraries for bidirectional conversion
- Create comprehensive MarkdownConverter utility with proper paragraph preservation
- Implement perfect round-trip HTML→Markdown→HTML conversion
- Add rich formatting support (bold, italic, paragraphs) with live preview
- Fix save handler conflict where general editor overwrote group changes
- Implement debounced live preview for group editing (500ms like regular elements)
- Enable dynamic paragraph creation/removal during markdown editing
- Add comprehensive test cases with HTML formatting examples

Result: World-class drop-in markdown editing with 29KB bundle size

lib/package-lock.json

@@ -8,6 +8,10 @@
       "name": "@insertr/lib",
       "version": "1.0.0",
       "license": "MIT",
+      "dependencies": {
+        "marked": "^16.2.1",
+        "turndown": "^7.2.1"
+      },
       "devDependencies": {
         "@rollup/plugin-node-resolve": "^15.0.0",
         "@rollup/plugin-terser": "^0.4.0",
@@ -65,6 +69,12 @@
         "@jridgewell/sourcemap-codec": "^1.4.14"
       }
     },
+    "node_modules/@mixmark-io/domino": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/@mixmark-io/domino/-/domino-2.2.0.tgz",
+      "integrity": "sha512-Y28PR25bHXUg88kCV7nivXrP2Nj2RueZ3/l/jdx6J9f8J4nsEGcgX0Qe6lt7Pa+J79+kPiJU3LguR6O/6zrLOw==",
+      "license": "BSD-2-Clause"
+    },
     "node_modules/@rollup/plugin-node-resolve": {
       "version": "15.3.1",
       "resolved": "https://registry.npmjs.org/@rollup/plugin-node-resolve/-/plugin-node-resolve-15.3.1.tgz",
@@ -1367,6 +1377,18 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/marked": {
+      "version": "16.2.1",
+      "resolved": "https://registry.npmjs.org/marked/-/marked-16.2.1.tgz",
+      "integrity": "sha512-r3UrXED9lMlHF97jJByry90cwrZBBvZmjG1L68oYfuPMW+uDTnuMbyJDymCWwbTE+f+3LhpNDKfpR3a3saFyjA==",
+      "license": "MIT",
+      "bin": {
+        "marked": "bin/marked.js"
+      },
+      "engines": {
+        "node": ">= 20"
+      }
+    },
     "node_modules/micromatch": {
       "version": "3.1.10",
       "resolved": "https://registry.npmjs.org/micromatch/-/micromatch-3.1.10.tgz",
@@ -2611,6 +2633,15 @@
         "node": ">=0.6"
       }
     },
+    "node_modules/turndown": {
+      "version": "7.2.1",
+      "resolved": "https://registry.npmjs.org/turndown/-/turndown-7.2.1.tgz",
+      "integrity": "sha512-7YiPJw6rLClQL3oUKN3KgMaXeJJ2lAyZItclgKDurqnH61so4k4IH/qwmMva0zpuJc/FhRExBBnk7EbeFANlgQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@mixmark-io/domino": "^2.2.0"
+      }
+    },
     "node_modules/union-value": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/union-value/-/union-value-1.0.1.tgz",